Fluid flow compressors

ABSTRACT

A fluid flow compressor includes means for arbitrarily obstructing part of the flow area of the compressor duct to improve the compressor efficiency at low flow velocities. The means comprises a ring arranged in the duct and capable of being expanded in the manner of a circlip by a cam arrangement.

United States Patent [191 Snell 1 Dec. 18, 1973 1 1 FLUID FLOWCOMPRESSORS [75] lnventor: Leonard Stanley Snell, Bristol,

England [73] Assignee: Secretary of State for Defence in Her BritannicMajestys Government of the United Kingdom of Great Britain and NorthernIreland, Whitehall, London, England [22] Filed: Dec. 3, 1971 [21] Appl.No.: 204,618

[30] Foreign Application Priority Data Dec. 17, 1970 Great Britain59,871/70 [52] US. Cl. 415/159, 415/151 [51] Int. Cl. 1. FOlb 25/02 [58]Field of Search 415/147, 149, 151, 415/ 159 Primary ExaminerC. .1.l-lusar Attorney-Richard K. Stevens et a1.

[57] ABSTRACT A fluid flow compressor includes means for arbitrarilyobstructing part of the flow area of the compressor duct to improve thecompressor efficiency at low flow velocities.

The means comprises a ring arranged in the duct and capable of beingexpanded in the manner of a circlip by a cam arrangement.

5 Claims, 5 Drawing Figures SHEET 1 U? V 4 PATENTED DEC 1 8 I975PATENTED mu: 1 8 I915 SHEET 38F 4 Y PAIENTEUDEEWIBH amass FIG. 5

FIG. 4

- FLUID FLOW COMPRESSORS This invention relates to fluid flowcompressors and particularly to their use in gas turbine engines.

It is well known in the art of gas turbine engines to experiencedifficulty in first starting an engine.

A starter is provided for rotating the compressor up to the engine speedat which the engine becomes selfsustaining. The higher thisself-sustaining speed is the more bulky and cumbersome is the starter.

The problem of starting a gas turbine engine at low rotational speedsoriginates in the design of the compressor.

The angles of incidence and attack of successive rotors and stators in acompressor are generally calculated on the basis of the gas velocitythrough them at -the cruise r.p.m., this is considerably higher than thestarting rpm. and it follows that the gas velocity through the bladingat cruise is also considerably higher than at the starting r.p.m.

During starting theaforementioned angles of incidence and attack areincompatible with the low gas velocity associated with the lowrotational speed of the compressor at the starting rpm. and theefficiency of the compressor suffers. In practice this means the enginewill not start until the compressor can be rotated at a higher speed.

Several approaches have been followed in order to eliminate thisproblem. It is well known for example to design a compressor of whichthe blades of the early rotor and or stator stages can be rotated abouttheir span axes, so as to alter the angles of incidence and attack. Itis further known to bleed off a quantity of air after the first two orthree rotor stages in this way increasing the mass flow through theearly stages and boosting'the gas velocity.

A third approach relies on blocking off part of the compressor duct toincrease the gas velocity through the blades, in the past this hasrequired complex and heavy arrangements to effectively obstruct parts ofthe duct.

According to the present invention a compressor for a gas turbine enginecomprises radially inner and outer wall elements together defining anannular compressor duct, a substantially annular structure at leastpartly housed within one of the wall elements, said structure beingsplit to form two ends and there being provided means operable on thestructure to produce relative movement between the two ends to project apart of the structure into the duct for varying the cross sectional areathereof.

A particular embodiment of the invention will now be described withreference to the accompanying drawings by way of example only, whereinFIG. 1 shows a longitudinal section through a gas turbine engine havingtwo spool axial flow compressor embodying the invention,

FIG. 2 shows the use of an expanded annular structure to obstruct thecompressor duct of the gas turbine engine,

FIG. 3 is a detailed view of a longitudinal section through part of acompressor embodying the invention.

FIG. 4 shows an axial view. of a particular annular structure and linkoperating means for expanding it in the manner of a circlip.

FIG. 5 is an enlarged drawing of the link operating means of FIG. 4.

Referring to FIG. 1 a gas turbine engine comprises a low pressurecompressor 11, a high pressure compressor 12, a combustion system 13 andhigh pressure (h.p.) and low pressure (l.p.) turbines 14, 15 connectedto their respective compressors by coaxial shafts 16 and 17. The h.p.and l.p. shafts 16 and 17 are each supported and constrained by twobearings respectively 18, 19 and 21, 22. At the entry to the h.p.compressor is provided an annular structure 23 which is completelycontained in a cavity 24 (illustrated in FIG. 2) except during thestarting of the engine when by means later explained its function is toobstruct part of the compressor duct 25. The compressor duct 25 is ofannular form and is defined by radially inner and outer wall elements30, 40 respectively.

Air delivered by the l.p. compressor 11 is received by the h.p.compressor 12. Substantially the same mass of air as delivered by thel.p. compressor will pass the annular structure 23 as would otherwiseflow through the whole duct 25. This raises the gas velocity across thefirst few stages 21 of the h.p. compressor.

The cavity 24 forms an integral part of the structure 20 supporting thefront bearing 18 of the h.p. shaft 16 the structure 20 also doubles asan inlet guide vane 26 for the h.p. compressor 12.

The detailed construction of FIG. 3 shows the front of the h.p. shaft 16supported by a bearing 18.

Attached to the h.p. shaft 16 is the first stage rotor 27 of the h.p.compressor 12, immediately in front of the rotor 27 is the bearingsupport structure 37 which also houses the shaft 28. The shaft 28 isformed integrally with the cam 29. The annular structure 23 is split atone point to form two ends 42, 43. The two ends 42, 43 are attached tothe two eyes 31, 32 respectively of the link 29 by pins 33 and trunnions34. The link 29 is operable on the annular structure 23 to producerelative movement between the two ends 42, 43 to change the radius ofthe annular structure 23. When the radius of the annular structure isincreased a part 44 of it will project into the duct 25 thus varying thecross-sectional area of the duct.

The increase and decrease of radius of the annular structurecorresponding to relative movement of the two ends 42, 43 may beconsidered analogous to the operation of a circlip and is illustrated inFIGS. 4 and 5. The annular structure 23 is equipped with flanges 35which restrict its radially outward movement by abutting with faces 36on the bearing support 37.

In the event of the circlip breaking it merely collapses harmlessly intothe cavity 24 and is prevented from passing through the engine and thuscausing subsequent damage by the flanges 35 being restricted by thefaces 42 on the bearing support structure.

Rotation of the shaft 28 is effected by a lever 38 attached to the shaft28 by a spline 41 working in the support structure, the rotation isillustrated by the arrow 39.

The mechanical advantage of the shaft 28 and link 29 system is such thatonly a small angular rotation of the shaft 28 is necessary to effect asubstantial increase in the radius of the annular structure 23. Theattachment of the cam 27 to the circlip 23 is more readily seen fromthis diagram.

The annular structure and link mechanism may of course be situated atother positions along the compressor duct 25, and may also beincorporated into the radially outer wall element 40. This latterembodiment requires that the annular structure is contracted by the linkin order to block off a part of the compressor duct.

It is sometimes desired to obstruct part of the crosssectional flow areaof a compressor duct at operating conditions other than starting. It mayfor example be useful in preventing surge phenomena whichcharacteristically occur at certain rotational speeds typical of theparticular engine, its installation and operating conditions. It mayalso be particularly desired to partially bstruct a sector of thecross-sectional flow area of a compressor duct and it will beappreciated that the invention may be applied to the above situations bymounting it in a position other than its normal position concentric withthe axis of the compressor duct.

The annular structure can be used on its own or in conjunction withother effective procedures, for boosting the gas velocity through acompressor.

I claim:

1. A compressor for a gas turbine engine comprising radially inner andouter annular wall elements together defining an annular compressorduct, a substantially annular structure at least partly housed withinone of the wall elements, said structure being split to form two ends,and there being provided means operable on the structure to producerelative movements between the two ends to project a part of thestructure into the duct for varying the cross-sectional area thereof.

2. A compressor according to claim 1, wherein the substantially annularstructure is mounted concentrically with respect to the compressor duct.

3. A compressor according to claim 1, wherein the substantially annularstructure is a circlip.

4. A compressor according to claim 1, wherein the substantially annularstructure is provided with a flange arranged to abut one of the wallelements of the duct to restrict the radial movement of the structure.

5. A compressor according to claim 1, wherein the means operable on thestructure to produce relative movement between the two ends thereofcomprises a link, means for connecting the link to the ends of thestrcutre and means for rotating the link.

1. A compressor for a gas turbine engine comprising radially inner andouter annular wall elements together defining an annular compressorduct, a substantially annular structure at least partly housed withinone of the wall elements, said structure being split to form two ends,and there being provided means operable on the structure to producerelative movements between the two ends to project a part of thestructure into the duct for varying the cross-sectional area thereof. 2.A compressor according to claim 1, wherein the substantially annularstructure is mounted concentrically with respect to the compressor duct.3. A compressor according to claim 1, wherein the substantially annularstructure is a circlip.
 4. A compressor according to claim 1, whereinthe substantially annular structure is provided with a flange arrangedto abut one of the wall elements of the duct to restrict the radialmovement of the structure.
 5. A compressor according to claim 1, whereinthe means operable on the structure to produce relative movement betweenthe two ends thereof comprises a link, means for connecting the link tothe ends of the strcutre and means for rotating the link.